It was the first summer of the Civil War, and everyone thought it would be the last. Hundreds of thousands of Americans converged on train platforms and along country roads, waving handkerchiefs and shouting goodbyes as their men went off to military camps. In those first warm days of June 1861, there had been only a few skirmishes in the steep, stony mountains of western Virginia, but large armies of Union and Confederate soldiers were coalescing along the Potomac River. A major battle was coming, and it would be fought somewhere between Washington, D.C., and Richmond.

In the Union War Department a few steps from the White House, clerks wrote out dispatches to commanders in California, Oregon and the western territories. The federal government needed army regulars currently garrisoned at frontier forts to fight in the eastern theater. These soldiers should be sent immediately to the camps around Washington, D.C.

In New Mexico Territory, however, some regulars would have to remain at their posts. The political loyalties of the local population—large numbers of Hispano laborers, farmers, ranchers and merchants; a small number of Anglo businessmen and territorial officials; and thousands of Apaches and Navajos—were far from certain. New Mexico Territory, which in 1861 extended from the Rio Grande to the California border, had come into the Union in 1850 as part of a congressional compromise regarding the extension of slavery into the West. California was admitted to the Union as a free state while New Mexico, which was south of the Mason-Dixon Line, remained a territory. Under a policy of popular sovereignty, its residents would decide for themselves if slavery would be legal. Mexico had abolished black slavery in 1829, but Hispanos in New Mexico had long embraced a forced labor system that enslaved Apaches and Navajos. In 1859 the territorial legislature, made up of predominantly wealthy Hispano merchants and ranchers with Native slaves in their households, passed a Slave Code to protect all slave property in the Territory.